1. Field of the Invention
The present invention relates to a red phosphor leading to sufficient luminescent at an image display devices driven with low voltages, such as field emission devices (FEDs), which is obtained using a conductive luminescent material, and a method for preparing the same.
2. Description of the Related Art
A conductive layer such as aluminum (Al) layer, which are used in cathode ray tube (CRTs), cannot be employed in FEDs, which are operated with a relatively high current density and low voltage. For this reason, use of commercially available phosphors in FEDs causes accumulation of space charges on the surface of the phosphors, and thus luminescence is lowered. This problem has been considered to be serious in applying phosphors to devices operating with low voltages. In order to avoid formation of potential barriers by space charges, a conductive material such as indium oxide or tin oxide is mixed with a phosphor or is coated on a phosphor to enhance the conductivity. Also, another approach suggests use of a conductive material as a host of phosphor to enable excitation with low voltages.
A technique of adding or coating indium oxide or tin oxide was disclosed in U.S. Pat. Nos. 4,027,192; 4,208,612; and 4,208,613, and JP No. 10212473. These materials have been applied as simple conductive materials in vacuum fluorescent displays (VFDs) after verification. However, the effect of the materials on luminance at FEDs is unclear yet. Reportedly, good results are obtainable only when the driving voltage is 400V or less (H. Kominami et al., xe2x80x9cLow Voltage Cathodoluminescent Properties of Phosphors Coated With In2O3 by Sol-gel Method,xe2x80x9d Applied Surface Science 113/114 (1997) 519-522, and R. Durny et al., xe2x80x9cSemiconductors,xe2x80x9d xe2x80x9cSuperconductors,xe2x80x9d xe2x80x9cMagnetism,xe2x80x9d xe2x80x9cOptics and Quantum Electronics,xe2x80x9d xe2x80x9cOptical Properties of Condensed Matter,xe2x80x9d xe2x80x9cStructure and Mechanical and Thermal Properties of Condensed Matter,xe2x80x9d xe2x80x9cSurfaces, Interfaces, and Films,xe2x80x9d and xe2x80x9cInstrumentation, Measurement, and Fabrication Technology,xe2x80x9d Japanese J. of Appl. Phys. Vol. 35 (1996) Pt. 2, No. 12A). Thus, there is a need for further enhancing luminance for FEDs.
To solve the above problems, it is an objective of the present invention to provide a phosphor having effective emission at low voltages, and a method for preparing the same, in which a material having both conductive and luminescent properties is mixed with a red phosphor, or the material is coated on a phosphor, to minimize accumulation of space charges with enhanced conductivity and decreased drop of luminance.
According to an aspect of the present invention, there is provided a red phosphor having effective emission at low voltages, the red phosphor mixed with a conductive luminescent material expressed by (Sn1-xEux)O2, in a predetermined concentration.
Preferably, the red phosphor is coated with the conductive luminescent material expressed by (Sn1-xEux)O2. Preferably, the concentration of the conductive luminescent material expressed by (Sn1-xEux)O2 mixed with or coated on the red phosphor is in the range of 5xc3x9710xe2x88x922 to 5% by weight. Preferably, in the expression of (Sn1-xEux)O2, x is a value from 0.0005 to 0.2.
According to another aspect of the present invention, there is provided a method of preparing a red phosphor having effective emission at low voltages, using a conductive luminescent material, the method comprising: (a) synthesizing a red phosphor Y2O3:Eu as a base phosphor, immersing the red phosphor into an ammonium hydroxide aqueous solution, and adding and stirring a solution containing Eu3+ and Sn4+ salts to the solution to form a precipitate on the red phosphor; (b) filtering and drying the precipitate; and (c) firing the dried precipitate at a temperature of 700 to 800xc2x0 C. to coat SnO2:Eu on the red phosphor.